3d Print an Artificial Muscle Robot Hand

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Here is an artificial muscle robot hand that could eventually be used as a prosthetic replacement for a human hand. The "bones" are 3d printed in PLA and the silicone rubber artificial muscles and skin were cast in 3d printed molds.

While most makers are not likely to want do duplicate this hand, some of the techniques used here might be found to be useful for other kinds of robots and casting.

Intro pic shows the hand holding a raspberry.

At this point, the soft robot fingers are a bit wobbly. I am working on adding an arm that will have room for more hand muscles. They would use tendons to open the fingers and balance the grasping muscles. This should stabilize the fingers and give a more natural movement.

The short video shows some of the movements of the robot hand.

Supplies:

Step 1: How It Works

This is an air powered hand that uses silicone artificial muscles as actuators. They are powered by compressed air. This gives it a soft touch that is much more human friendly than most robot hands. The artificial muscles are also very inexpensive to produce compared to traditional gear motors and servos.

Ideally, the whole thing would be printed completely in a 3d printer that printed a hard plastic and a soft rubber that holds air pressure. Since I have no access to such a machine, the soft silicone parts had to be cast in 3d printed molds.

This is an open source project. Version 1.0 is mainly a proof of concept--prototype and there is lots of room for improvements. While not near as strong as a human hand, it is strong enough to hold a cup of coffee or carry a briefcase.

Step 2: Materials

I used a Makerbot Replicator 2 to print the parts and molds in PLA plastic. Step 2 pic shows the 3d printed and cast parts used to make the robot hand.

Dragon Skin 20 pourable silicone from Smooth-On.com. Other kinds of silicone can be used, but this has very good flexibility and pour ability for casting the artificial muscles and skin.

Step 4: Casting Artificial Muscles in Break-Apart Molds

An entire finger is cast in one 3d printed mold. This produces a soft robot finger that contains 3 muscles, soft bones between muscles, and a fingertip. It also contains two molded-in air channels that provide power to the muscles. Once cast and de-molded, the embedded separator fins are bent until they break away from the air bars. The air bars are then slid out to leave air channels. Silicone tubing is then glued to the air holes with silicone caulk or Oogoo. The Top separator fins are left embedded in place and do not hinder the muscle movement.

Three fingers are all the same size. The pinky finger is scaled down to 90 per cent of full size.

Break-Away Molds The finger molds are designed to break apart into several pieces (pic 4) in order to make it easier to de-mold the silicone fingers. I first experimented with thicker reusable molds, but found that no matter what kind of release I used, it was extremely difficult to remove the cast silicone from the mold. They just have too much surface area. Half the time, I had to destroy the mold to get the casting out. So I ended up making very thin molds with break lines to make it easier to de-mold. No release is necessary.

Three Part Molds The two lower thumb muscles were cast in two bottom molds that are taped together. A top piece with separator fins completes the mold.

Casting Silicone With A Vacuum Chamber Unfortunately a vacuum chamber is required to cast good silicone artificial muscles. Without it, the muscles would have bubbles that leak air even under low pressure. A vacuum pump is used by subjecting the silicone mix to a vacuum for 2 or three minutes before it is poured into a mold.

Using MoldsThat Dissolve I also experimented with 3d printed molds made of HiPS filament that dissolves in Limonene. While it dissolves well enough, the Limonene shrinks and hardens the silicone to an unacceptable degree. It also permanently saturates the silicone making it impossible to glue to it. Acetone works better, but to a lesser degree, it also shrinks and hardens the silicone.

I have begun experimenting with PVA filament that dissolves in water, but have not fully worked it out yet.

Step 5: Thumb Muscles

Step 5 pic shows the two lower thumb muscles. I did not get the pivot point correct. The back muscle should have rotated toward the palm. Something to fix in the next version.

Step 6: Casting the Removeable Robot Skin

The skin on the back of the hand is designed to slot into grooves in the bone structure. This makes it possible to peel it back to access the inner muscles and bones.

Skinmold.stl is used to cast the skin. A piece of acrylic sheet is put on top of the mold and weighted after pouring to keep the skin thin.

The skin pads were cast in forms and were later glued to the PLA palm plate and thumb muscles using Oogoo. To get good adhesion a thin coat of pure silicone caulk has to first be put on the PLA where the pads go. Let it dry overnight and then use Oogoo to glue the pads on. Oogoo by itself will not stick very strongly to the PLA.

If you just want to test some air muscles without a controller, a 60cc syringe with tubing works well. It can provide up to 30 psi.

Step 8: The Robot Neuron Schematics

Here is the schematic for the the master robot neuron that controls the pneumatic valves that power the artificial muscles. There are basically three Picaxe micro-controllers that are serially networked. A master neuron sends commands to the two actuator neurons that control the valves.

Step 9: The Robot Neuron Code

Here is the Picaxe code that controls the robot neurons. A universal TV remote control set up to use Sony code can be used to control the individual muscles. Muscle sequences can also be activated with the remote.

if b1 = 1 then act13 if b1 = 2 then act13 if b1 = 3 then act13 if b1 = 4 then act13 if b1 = 5 then act13 if b1 = 6 then act13 if b1 = 7 then act13

if b1 = 8 then act8 if b1 = 9 then act9 if b1 = 10 then act10 if b1 = 11 then act11 if b1 = 12 then act12 if b1 = 13 then act13 if b1 = 14 then act14 if b1 = 16 then vid1 if b1 = 17 then vid2 if b1 = 116 then closehand if b1 = 21 then powervac5

Step 10: Other Possibilities

Foot Powered Compressor One of the advantages of using air powered muscles is that it should be possible to design an air pump that fits in or on a shoe. This could be used to pressurize a small flat backpack tank while walking. This could keep a prosthetic hand and arm powered in a fairly unobtrusive way.

Finger Tendons One thing lacking in this first version of a robot hand is tendons and muscles to pull the fingers open. Right now it relies on the stiffness of the fingers, so it is a bit floppy. I experimented with various tendons (silicone elastic bands) and ligaments to open the fingers. Unfortunately, they severely hampered the grasping power of the fingers.

Once an arm is built, it could house four or five air muscles that pull on tendons to open the fingers. I have also been working on pull type muscles that use tendons.

Higher Pressures I used surplus valves for the muscle controller and they can only hold about 9 psi. I am currently testing some 3d printed valves I designed that will work up to 30 psi. This will increase the grasping power and speed of the fingers considerably.

Robot Prosthetic Arm I have started working on a human sized arm, but it is not yet ready. I am working on making my own lightweight valves that will fit in the arm.

I've always wanted to make a robotic hand that mimics my movement and i came across this one. If i want it to mimic my movement, would I have to reprogram this whole thing because I don't know how to program a picaxe?

Our Muscles can only pull and can't push so for each degree of freedom we have, we have a pair of muscles (called skeletal muscles) that alternate pulling in opposite directions. Apparently your system can only push, if you can implement two muscles for every DOF in your system, it'll be more than great additionally you'll have more accurate positioning.

1) there are no muscles in fingers nor thumb, all the muscles are located on the palm, back of the hand and, mainly, in the forearm.

2) hydraulic (air) is slow, weak and not so precise, oil would probably be a better choice. Electro-magnetic muscles would be more accurate and even more powerful, slicing plates of electro-magnets between rubber and alternating polarity would make it a lot more realistic, powerful (your limit is when you start getting arcs between the plates, they tend to burn the rubber) -- and fast. If your goal is to make a prosthetic hand, you probably want to match your set of muscles with the (existing) set of nerves (you'll probably need a little more inputs and outputs than what can be found on an Arduino, though)

Very cool - so what is powering the air?? I saw the other gripper but I wonder about calling it a robot as to me that implies that there are sensors for inputs and some kind of automated output (inflation here). How do you do that automation part? Thanks!